rust: types: remove `Either<L, R>`

rust: types: remove `Either<L, R>`

This enum is not used. Additionally, using it would result in poor
ergonomics, because in order to do any operation on a value it has to be
matched first. Our version of `Either` also doesn't provide any helper
methods making it even more difficult to use.

The alternative of creating a custom enum for the concrete use-case also
is much better for ergonomics. As one can provide functions on the type
directly and users don't need to match the value manually.

Signed-off-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250519124304.79237-1-lossin@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>